Diodes are semiconductor devices made of two oppositely doped semiconductor layers which are characterized by the ability to block high voltage in the reverse direction with very low leakage current and carry high current in the forward direction with low forward voltage drop. They are therefore unidirectional switches which allow signal and power to pass in one direction but not the other. They are widely used in power electronic circuits to provide the functions for freewheeling, rectification, and snubbing in converters, inverters, motor controls, switch mode power suppliers, power factor correction, inductive heating, welding, uninterruptible power supplies and many other power conversion applications.
These power diodes, including one subgroup referred to as FREDs (Fast Recovery Epitaxial Diodes) and another subgroup using bulk semiconductor material, usually consist of an active area and a peripheral edge region. The active area in the center of the semiconductor device carries high current in the forward conduction and blocks high voltage in the reverse direction with low leakage. The peripheral edge region must also block the same high reverse voltage with equally low reverse leakage current through the use of guard-rings, bevels, grooves and other surface field spreading structures to support the high voltage in reverse blocking. For purposes of illustration, the drawings and ensuing discussions focus primarily on the FREDs. However, the inventive principles apply equally to all types of diodes as well as devices possessing diode-like structures as part of the device construction.
In modern power conversion applications, these diodes are used in conjunction with other high current, high voltage semiconductor devices such as high frequency Insulated Gate Bipolar Transistors (IGBTs) and power MOSFETs. In such high frequency and high power applications, particularly in power electronic circuits with inductive loads, the power diodes are required not only to have high breakdown voltage and high current capability but also to have high ruggedness.
FIG. 9 shows a common boost circuit where a fast recovery diode can be used. The lower input voltage V1 is converted to a higher output voltage V2 to drive a load. V1 is represented here as a DC voltage. In extended applications, it can be a rectified voltage off the AC line. As shown, the circuit uses a single power switch Q and a single diode D. L and C are an inductor and a capacitor. When the power switch is conducting, it stores energy in the inductor. When it is turned off, the stored energy is diverted into charging the capacitor C. The value of V2 is dependent on the switching frequency, the duty ratio and the resistance and capacitance in the circuit. The maximum voltage applied across the diode D is about the difference between the output voltage V2 (neglecting diode forward voltage drop) and voltage drop across the power switch. When the diode is not rugged, this voltage is kept below 60–70% of the diode rated voltage so that noise and spikes inherent in the environment will not accidentally spike above the diode avalanche voltage, causing it to fail. This imposes an undesirable guard-band condition. If rugged diodes are available, one can either raise the output operating voltage or use a diode with a lower blocking voltage in the same circuit. In the latter case, a lower voltage will have associated with it a lower forward voltage drop, therefore, lower conduction loss, and furthermore a lower recovery loss. The circuit layout will generally not change when a rugged diode is used. The main difference is in improved circuit reliability and efficiency.
The above-mentioned ruggedness is usually measured by its Unclamped Inductive Switching (UIS) capability, i.e. the ability of the device to go into avalanche and dissipate all the energy stored in the inductive load without suffering any damage. For conventional diodes (including FREDs) which are most popular and available in the present market, it is easy to get high voltage and high current ratings, e.g. 1200V/200 A, but it has not heretofore been possible to get high UIS capability. The highest UIS capability rating we can find so far in the market is 20 mJ at 1 A for Stealth Diodes (Trade Mark of Fairchild Semiconductors) rated at forward currents of 8 A and 15 A with 600V reverse blocking voltage and 30 A with 1200V reverse blocking voltage. (See referenced Product Data Sheets for these devices from Fairchild Semiconductors). Such low UIS energy capability is hardly adequate to prevent the diodes from being damaged in the presence of voltage spikes, let alone to protect other devices in such circuits. Therefore, to have high UIS capability for diode products is an important objective.
The ensuing discussions and illustrations are given for a P+N diode structure. It is obvious that the specific ideas of the invention apply equally well to an N+P diode if the polarities of the appropriate dopants are reversed.
Conventional diodes are made by introducing typically a P-type dopant such as boron, gallium, or aluminum into an N-type semiconductor substrate and diffuse it to an appropriate depth to create a PN junction. This P-dopant forms the central active area of the device for forward conduction. The substrate doping level and thickness of the N-type region are adjusted so as to obtain the desired blocking voltage and the desired forward voltage since the product of forward current and forward voltage measures power loss by the diode as it controls electric power. At the periphery, different voltage spreading and electric field reduction techniques are commonly used to withstand the reverse blocking voltage. The P-type doping level and junction depth are varied dependent upon the desired blocking voltage and the voltage blocking scheme used. In general, sufficient P dopant is introduced so that, up to the avalanche voltage, there remains substantial P-dopant to prevent “reach-through” conduction to the electrode connected to the P region. Design of the P-dopant profile can be made by considering charge balance on the P side and the N side of a PN junction under the designed reverse voltage. For example, it is generally sufficient to block 1200V with a P-diffusion of about 5 to 8 μms (microns) employing multiple plane guard-rings and field plates at the device periphery with a surface doping concentration anywhere from 1.0E16/cm3 to 1.0E19/cm3. For a 5000V diode, the same general guidelines for substrate doping and the amount of P-dopant still apply. It usually takes a deeper P-diffusion depth, anywhere from 30 to 90 μms, to provide sufficient P-type charge for such high voltage. Furthermore, it requires the use of single or double beveling to reduce electric field at the device periphery to sustain the desired blocking condition.
Prior art conventional devices have low ruggedness because the avalanche conditions of these devices occur in a limited area at the periphery of the active P−N junction, as shown in FIG. 1(a). Because avalanche occurs in a limited area, and avalanche current is confined to the limited area, the ability of the device to dissipate energy under the avalanche condition without causing damage to the device is also limited.